runtime: don't crash if no p in kickoff
runtime: don't crash if no p in kickoff

The kickoff function for g0 can be invoked without a p, for example
from mcall(exitsyscall0) in exitsyscall after exitsyscall has cleared
the p field. The assignment gp.param = nil will invoke a write barrier.
If gp.param is not already nil, this will require a p. Avoid the problem
for a specific case that is known to be OK: when the value in gp.param
is a *g.

Change-Id: Ifc0451359a2b09a62bff51159186a10900bd9f01
Reviewed-on: https://go-review.googlesource.com/46512
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend